Charge Diffusion Measurement in Fully Depleted CCD using 55Fe X-rays

Tight requirements on the Large Synoptic Survey Telescope point spread function (PSF) demand sensor contribution to PSF be both small and well characterized. The sensor PSF is determined by the lateral charge diffusion on the drift path from the photon conversion point to the gates. The maximum drift path occurs for photons converted at the window, for blue optical photons in particular. Charges generated at the window surface undergo "worst case" charge spreading and the blue optical PSF is used to characterize the sensor's PSF. Different techniques for charge diffusion characterization have been developed, each with its own systematics and measurement difficulties. A new way to measure charge diffusion using an X-ray source is presented. We demonstrate the effectiveness and limitations of our technique and discuss relation of charge diffusion value obtained with X-ray measurements to sensor PSF.